Conventional vehicle fuel systems have a liquid fuel tank with a single fill level. A filler pipe is adapted to receive a pump fuel nozzle, and feeds fuel into the tank interior. The air and fuel vapor mixture displaced by the entering liquid fuel must be allowed to exit the tank freely in order to prevent a pressure build up that would otherwise block the liquid fuel from entering. Free venting is typically provided by a vent pipe, the inlet of which rests at a predetermined level or height within the tank interior, and the outlet of which vents either directly to atmosphere, or, in proposed new designs, to a vapor storage canister. The height of the vent pipe inlet within the tank determines the tank fill level. When liquid fuel reaches and blocks the vent pipe inlet, pressure in the tank builds up, sending liquid fuel up the filler pipe and automatically shutting off the fuel nozzle.
The empty volume remaining between the surface of the liquid fuel and the top of the tank at the end of fill is needed in order to accommodate the fuel vapor that will inevitably form, and which would otherwise raise the pressure within the tank. Since more fuel will vaporize in hot than in cold weather, the residual volume must be made large enough to accommodate the hot weather condition. This means that the tank cannot be filled as high in colder weather as it otherwise could be. Tailoring fill level to the temperature would give more efficient utilization of the available tank volume.